This phrase is usually stamped on the back of professional electronics equipment, like monitors, VCR's, and such. It is rather stupid to stamp that there, since you can use such a monitor or VCR pretty much anywhere. If I want to buy one at the flea market and plug it into my VCR, I should damn well be able to.

That phrase is there to protect you from the product. The product may be dangerous in some way that a lay-person may not even conceive of.

For example, your flea market "Professional" monitor may be pumping out more (or stranger) radiation than is acceptable in the consumer version.